The present invention is related to a string core-free invisible zipper structure in which the extensible tolerance of the connecting section between the upper and lower leg sections of adjacent zipper teeth is reduced and the zipper teeth have better anti-pull strength and snapping ability. Therefore, without any string core passing through the zipper teeth, the zipper teeth still have sufficient strength. In addition, it is easier to sew the zipper and the manufacturing cost is reduced.
FIGS. 4 and 5 show an early zipper. After sewing, the seam of the zipper is exposed to an outer side of the zipper belt so that the appearance is poor. An invisible zipper has been developed to improve this. When sewn, the seam 12 is hidden in an inner face of the zipper belt to beautify the appearance. The invisible zipper includes a pair of zipper belts 1, with zipper teeth 2 continuously spiraled on one side of the inward folded edge of each zipper belt 1 and a slider slidably along the length of the inward folded edge for zipping the zipper on and off. The transverse gap 4 defined between the upper and lower leg sections 21, 22 of the zipper tooth 2 is equal to the inner diameter of the slider. Accordingly, after the zipper teeth are latched with each other, when subject to external force, the length of the connection sections 5 of the adjacent zipper teeth 2 still can absorb the increased tolerance caused by the shift of the transverse gap 4. Therefore, when the length of the connecting sections 5 of the zipper teeth 2 is increased, the snapping ability of the zipper teeth 2 is lowered or even totally lost. This leads to poor function of the zipper. An improved zipper has been developed. An auxiliary string core 6 is passed through the interior of the zipper teeth 2 and sewn or stitched on the zipper belt 1 by a yarn together with the upper and lower leg sections 21, 22 and the connecting sections 5. The string core 6 is retained between the upper and lower leg sections 21, 22 to reduce the gap therebetween. When the zipper teeth 2 are subject to external force, the string core 6 is compressed and flattened, the zipper belt 1 between the upper and lower leg sections 21, 22 is naturally shifted to the connecting sections 5. This leads to unstability of the zipper teeth. The string core further can press the connecting section 5 and prevent the same from being tensioned by external force and fix the zipper teeth 2 on the zipper belt 1 so as to keep a constant gap between the zipper teeth 2 and achieve a desired buckling effect. However, the string core 6 leads to higher cost and difficulty in manufacturing. Also, the thickness of the produce is increased. This affects the flatness of the appearance of the product. Furthermore, when repeatedly washed or subject to heat, the string core tends to deform or expand. Therefore, when pulling the slider, the slider will be obstructed or even damaged. This shortens the using life of the product.
In another type of invisible zipper, the lower leg sections of the zipper teeth are sewn by way of channel sewing. This is disadvantageous in that when the zipper teeth are pressed on front face, the zipper teeth will retract and disengage from each other during snapping.
It is therefore a primary object of the present invention to provide a string core-free invisible zipper structure including a pair of zipper belts and zipper teeth windingly extended and continuously lengthwise sewn along the inward folded edge of each zipper belt. The gap between the upper and lower leg sections of the zipper tooth is gradually converged from the head section of the zipper tooth to the heels of the upper and lower leg sections, whereby the extensible tolerance of the connecting section between the upper and lower leg sections of adjacent zipper teeth is reduced and the deformation of the zipper teeth caused by lateral pull is reduced. Therefore, the zipper teeth have sufficient buckling strength and the possibility of disengagement of the zipper teeth is minimized. Therefore, the string core passing through the zipper teeth is no more necessary for reinforcing the anti-pull strength of the zipper teeth. Without any string core passing through the zipper teeth, the zipper teeth still have sufficient strength. In addition, it is easier to sew the zipper and the manufacturing cost is reduced.
The present invention can be best understood through the following description and accompanying drawings wherein: